The present invention generally relates to air cycle Environmental Control Systems (ECSs) and, more specifically, to an improved air cycle ECS and improved method of conditioning water vapor bearing air while minimizing energy losses that might otherwise occur during water vapor removal.
ECSs are used to provide a supply of conditioned air to an enclosure, such as an aircraft cabin and cockpit. An air cycle ECS typically operates on a flow of bleed air taken from an intermediate or high pressure stage within a jet engine having multi-compression stages or from an auxiliary power unit (APU) having a compressor specifically designed as a source of compressed air to the aircraft air conditioning system. All air compressed by the engine or APU is initially ambient air which may contain substantial amounts of moisture while operating at low altitudes. One important function of the ECS is to remove most of the moisture that would otherwise condense and be supplied as unwanted liquid droplets in the cool supply of conditioned air to the cabin.
Two main ECS types have been used most recently and are considered state of the art for an efficient air cycle ECS. These ECSs are referred to as the 3 wheel and 4 wheel high pressure water separation (HPWS) systems. The reference to 3 wheels relates to the fact that three rotating aerodynamic impellers (namely, one turbine, one compressor and a fan) are tied to one another by a common drive shaft. The bleed air is usually pre-cooled within a single, primary heat exchanger where the heat is rejected to ambient air, and then flowed to the single compressor. The fan moves the ambient air through the heat exchanger while the aircraft is on the ground and no air speed is available to push the air through such exchanger. Because water is removed before the air flow reaches the turbine, the flow is at a relatively high pressure created by the single compressor. Hence, the term "high pressure water separation" is used.
Typically following compression in the 3 wheel HPWS system, an ambient air secondary heat exchanger, a reheater, and a condenser are employed for air cooling and water condensation. The secondary ambient air heat exchanger cools the air back to near ambient temperature. Next, the reheater further cools the air and the condenser then completes the cooling process. In the condenser, a temperature is reached where most of the water content has to condense into a liquid form. Some prior condensation, however, takes place in the secondary heat exchanger and the reheater.
After final condensation in the condenser, the liquid water is removed by a water extractor. The resulting dehumidified air flows to the reheater where it cools the incoming moisture laden bleed air and, in turn, the dehumidified air is reheated. The reheated dehumidified air, which contains essentially no liquid moisture, is then supplied to the single turbine for expansion and cooling. A cool, expanded air from the turbine is then flowed into the condenser where the incoming moisture laden bleed air is cooled. In the condenser, the cool expanded air absorbs a heat load equal to the heat of condensation removed from the moist bleed air and the sensible cooling load corresponding to the temperature reduction imparted to the bleed air while in heat exchange in the condenser. The air from the turbine, which has now been warmed in the condenser, is then directly supplied to the cabin.
A feature of the current 3 wheel HPWS system is that the reheater is used to recover part of the cooling load necessary for water condensation. This is because the turbine can transform the extra energy represented by the combined pressure and temperature of the incoming reheated, dehumidified air into mechanical energy (i.e., shaft power), and can deliver air at lower pressure and temperature. As air at higher temperature is input to the turbine after being warmed up in the reheater, the thermodynamic laws of expansion dictate that a larger temperature drop occurs in the turbine, as well as more turbine power. This extra cooling and mechanical power represent a partial "recovery" of the heat energy added to the flow in the reheater that is not passed on to the cabin. The recovery is, in fact, only minimal in the 3 wheel HPWS because there is only a partial amount of condensation that occurs in the reheater. The majority of the energy related to the water condensation process is exchanged in the condenser and is not returned to the turbine for recovery. Instead, such non-recovered energy finds its way directly into the cabin in the form of higher supply temperature
In a 4 wheel HPWS system, a second turbine (i.e., the fourth wheel) is added downstream of the condenser. With both the 3 and 4 wheel systems, the dehumidified air leaving the condenser contains extra heat exchanged in the condensing process. In the 4 wheel system, instead of adding entirely to the cabin supply temperature, that extra heat provides a higher energy level to the air entering the second turbine, and partial recovery of that energy can take place in the second turbine. At typical turbine pressure ratios and efficiencies, the 4 wheel system is generally more energy efficient than the above 3 wheel system.
Despite the general energy efficiency advantage of the prior 4 wheel HPWS design over the 3 wheel HPWS design, the former still has disadvantages. A drawback of the fan being tied to the compressor and turbines is that optimal operation of each of the 4 wheels must be balanced against each other, which usually means one or more of the 4 wheels operating at less than optimum levels. A further disadvantage is that the fan draws power from the system and thereby reduces the amount of available power for compression. With less compression, there is less of a pressure ratio drop for the turbines, which means less cooling capacity.
Additionally, in the prior 4 wheel HPWS, the temperature flow from the first turbine and into the condenser typically needs to remain above freezing. Such a temperature limitation imposes a pressure ratio expansion or temperature drop limitation across the first turbine. With a temperature drop limitation, there is an accompanying energy recovery limitation on the first turbine.
Both the 3 and 4 wheel systems usually remove water at a pressure level resulting from typical "low stage" port bleed from the engines, plus one subsequent stage of compression in the ECS itself. Higher water extraction efficiency could result from condensing at higher pressures, leading to drier air supply. Alternatively, higher condensing temperatures could be used if pressures were higher for the same water removal capability, which would reduce heat exchanger size and sensible heat energy penalties. To achieve higher condensing pressures, current systems would have to rely on higher bleed pressures, which are either unavailable from current APUs and engines or would be more costly in engine fuel usage.
Past attempts have been made in various ways to address the issue of higher pressures by calling for dual stages of compression in the ECS itself. For example, U.S. Pat. No. 3,289,436 uses both dual compressors and dual turbines with RAM heat exchangers intercooling after each stage of compression. This system, however, was designed for operation at high aircraft speeds (i.e., supersonic) with high bleed pressures and high temperatures from the engine. Because the RAM heat exchangers are in series for the RAM air, they result in large size and length. Further, the series arrangement of the turbines does not show any provision for condensing water. Indeed, the disclosed invention does not address the issue of water separation and extraction, a fundamental aspect of system efficiency.
U.S. Pat. No. 2,767,561 also shows the use of two stages of compression and expansion, but does not describe any particular heat exchanger configuration regarding the RAM air circuit. If the heat exchangers are used in parallel to the RAM flow, they will lead to large size and excessive ram flow usage. If used in series, the second intercooler will lead to elevated turbine inlet temperatures, which is unsuitable for adequate water extraction. In fact, the disclosed invention does not address water extraction. Designed for high speed and high engine bleed pressures, this system is not compatible with current needs for high efficiency, low fuel usage, and relatively low engine bleed pressures.
The prior use of two compressors but with only a single turbine is shown in U.S. Pat. No. 4,312,191. The first compressor is used to boost the pressure level of cabin air which is recirculated and later cooled by the system. However, this system describes a water separation method only at low pressure after the turbine expansion. Therefore, the system is less efficient at removing water than the current 3 and 4 wheels HPWS. Furthermore, the air cycle system is not self-powered, thus requiring complex mechanical drives.
As can been seen, there is a need for an improved ECS and improved method for higher pressure water condensation and extraction in an ECS, both of which increase efficiency. Specifically, there is a need to increase the pressure expansion ratio available to the turbines to provide greater cooling capacity. Another specific need is to lower the energy requirement for water condensation in the condenser and leave more energy for useful work. Likewise, there is a need to minimize energy losses incurred in the water condensation and removal process of an ECS. Specifically, there is a need for an ECS to reduce the amount of water condensation energy that is added in the form of heat to the supply stream to the enclosure. There is a further need for an ECS and method of condensation that provides more cooling capacity for a given sized system, or a smaller sized system for a given load. In particular, there is a need for heat exchanger configurations that not only minimize size, weight, and RAM air usage but also intercool between staged compressions.